It is observed while approaching 100% SoC stage, a so-called imbalance phenomenon indicated by one cell reaching the Max voltage value around 3.7 V. at the same time the voltage of other cell may only be 3.35 V which is the minimum value, thus giving around 350 mV variation.
At our 1st version of BMS software, a beeping and alarm will be triggered at individual cell voltage reaches 3.7 V. Therefore, at the end of charging stage, it is easy to trigger the mentioned beeping and alarm.
In our 2nd version of BMS software, we decided to take out this beeping and alarm on the foundation of data indicated as shown in Fig. 2, once the SoC reaches 100%, the charging step will stop. And afterwards the variation in voltage among cells drops immediately and dramatically as seen from the green line of Fig. 2. This period only lasts around 50 s as indicated from Fig.1.
The mechanism behind this phenomenon is so-called polarization of Li-ion battery. And it normally occurred at the end of charging stage while the SoC reaches range from 99%-100%. It is obvious at the other SoC stages, the variation of voltage is limited less than 50 mV, thus our engineer decided to remove the beeping and alarm eventually in our BMS software and will not affect the normal use. as we realised the battery beping at 100% would become annoying.
Fig. 1 Voltage vs Charge Time Plot
Fig. 2 V_max-V_min value as a function of SoC
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